The present invention relates to a process for producing a biaxially oriented polyester film excellent in thickness uniformity as well as in flat and slippery properties.
Oriented polyester film is produced by stretching an amorphous polyester film in machine direction and/or longitudinal direction. And oriented polyester film is provided with mechanical properties according to their application purposes by suitable selection of production conditions for desired film.
However, it has been difficult to prepare a film having desired properties simultaneously because obtaining some preferred properties will inevitably sacrifice other properties and/or productivities to some extent. Above all, thickness uniformity and flat and slippery properties are primary characteristics required for a film, therefore, a variety of attempts have been made for providing the film with all these properties simultaneously, but with unsatisfactory results hitherto.
Considering these problems, the present inventors, as the result of earnest investigations, found that a film excellent in both thickness uniformity and flat and slippery properties can be easily prepared by effecting machine direction stretching at a specific condition, and have eventually completed the present invention based on this finding.